fix: flow libdefs file naming

[maxmalov]

Currently, Flow is unable to pick bundled library definitions, since it can't match index.js.flow with final-form-arrays.cjs.js or final-form-arrays.es.js. So I borrowed build configuration from final-form repo https://github.com/final-form/final-form/blob/master/package-scripts.js#L58-L64 to resolve the issue

[codecov]

Codecov Report

Merging #52 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #52   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          159       159           
  Branches        34        34           
=========================================
  Hits           159       159           

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